1. Field of the Invention
This invention is in the field of motor controllers for electric motors, particularly those with variable speed and variable torque, such as switched reluctance, or SR, motors and permanent magnet, or PM motors, but is not limited to such motors; and more particularly relates to improvements in controlling the pulse width modulation (PWM) of the power drive signals that determine the speed and torque of variable speed electric motors.
2. Description of Related Art
Recent developments in power semiconductor devices such as power MOPPETS and insulated gate thyristors (IGT)s have led to the development of electronically commutated motors for use in applications requiring variable speed drive motors. Common examples of the types of electric motors the speed and torque of which are controlled by controllers which pulse width modulate the current flow through the power phase winding circuits of such motors, are SR motors and PM motors; however, the controller of this invention can be used with any electric motor that can be controlled by pulse width modulating the flow of electrical current through the motor's power phase windings. The cost and reliability of the pulse width modulation (PWM) controllers for electric motors compare favorably with those of more conventional controllers for variable speed motors.
Motors such as SR motors and PM motors conventionally have multiple poles on both the stator and rotor. In a SR motor, there are power phase windings on the stator poles, but no windings or permanent magnets on the rotor. Each pole of each pair of diametrically opposite stator poles of a SR motor have series connected windings that form an independent power phase winding. In a PM motor, permanent magnets are usually mounted on the rotor.
Torque to rotate the rotor is produced by switching current into each of the power phase windings in a predetermined sequence that is synchronized with the angular position of the rotor, to polarize an associated pair of stator poles. While generally the power phase windings are placed on poles of the stator, they can be placed on poles of the rotor if so desired. The resulting magnetic force attracts the nearest pair of rotor poles. In a SR motor, current is switched off in each power, or stator, phase winding before the poles of the rotor nearest the excited stator poles rotate past the aligned position. In such motors, the torque developed, while a function of the magnitude of the current flow in the stator windings, is independent of the direction of current flow so that unidirectional current pulses synchronized with the rotation of the rotor can be applied to the stator power phase windings by a convertor using unidirectional current switching elements such as thyristors or power transistors. The desired commutation of current through the stator phase windings can be accomplished by producing a rotor position signal by means of a shaft position sensor; i.e., an encoder, or resolver, for example, which is driven by the motor's rotor. The rotor position signal is applied to the motor controller.
The motor controller also typically has applied to it a signal indicating the desired direction of rotation of the rotor and a speed set signal indicating the desired angular velocity of the rotor which is typically measured in revolutions per minute (RPM). Such speed and direction signals may be controlled by a human operator or, more effectively, an automated control system. In addition, a rotor position signal, which is also known as the motor electrical (Me) signal; and a torque, or current, feedback signals are also applied to the motor controller. Current for each of the power phase windings of a SR motor is derived from a unidirectional power source, and each of the power phase windings is connected in series with a power transistor to control the flow of current through its associated power phase winding. The motor controller produces pulse width modulation
(PWM) power drive signals which are applied to the power transistors to turn them on and off. The timing of such current flows relative to the position of the rotor causes the rotor to rotate, and the order in which the power phase windings are energized determines the direction of rotation of the rotor.
The power drive signals applied to the power transistors in series with power phase windings are pulse width modulated (PWM) to maintain current levels through the power phase windings at a level to cause the rotor to rotate at the desired RPM while limiting the torque, or current, in the power phase windings to a predetermined maximum. It should be noted that the magnitude of the torque of a motor is a function of the magnitude of the current flowing through its power phase winding circuits. The magnitude of this current flow is sensed and used to produce a current, or torque, feedback signal which is applied to the motor controller. A prior art circuit for pulse width modulating the power drive signal for a SR motor is illustrated in FIG. 9 of U.S. Pat. No. 5,196,775.
A problem with prior art PWM motor controllers is that there is no fixed relationship between the frequency of the PWM power drive signals and the motor electrical, Me, or power phase commutation signals which results in a beat frequency (PWM-Me) that causes fluctuations at this beat frequency in the speed and torque of the motor. Such fluctuations in and of themselves are undesirable, and in addition they also increase the noise produced by a motor in which such fluctuations occur.